1. Field of the Invention
The present invention relates to a hydraulic control system for controlling hydraulic pressure that is supplied to pulleys provided in a belt-type continuously variable transmission for a vehicle.
2. Description of the Prior Art
Conventionally, as an example of continuously variable transmissions suitable for a vehicle, there is a belt-type continuously variable transmission wherein a V-belt is wound between a primary pulley and a secondary pulley.
Generally, in these continuously variable transmissions, a hydraulic pressure (hereinafter, primary pulley pressure) obtained by regulating a line pressure as an original pressure with a shift control valve is supplied to a cylinder chamber of the primary pulley, and a hydraulic pressure (hereinafter, secondary pulley pressure) obtained by reducing the line pressure as the original pressure with a pressure reducing valve is supplied to a cylinder chamber of the secondary pulley. The primary pulley pressure is increased or reduced with the shift control valve to change the groove width of the primary pulley, and radius ratio of the primary pulley and the secondary pulley is changed, whereby gear ratio (reduction ratio) of the vehicle such as an automobile is continuously controlled.
Rotation sensors detecting rotation speed of the pulleys are respectively provided in the primary pulley and the secondary pulley, and an actual gear ratio of the belt-type continuously variable transmission is calculated by calculating a ratio of the rotation speeds detected with these rotation sensors.
When trouble such as breaking occurs in a secondary pulley rotation sensor detecting rotation speed of the secondary pulley, a gear ratio calculated on the basis of the rotation speeds detected by the rotation sensors is abnormally increased (changed to a low speed side).
In this case, since it cannot be determined whether the gear ratio has been abnormally increased due to a slip that occurs in the V-belt wound around the respective pulleys, or whether the gear ratio has been abnormally increased due to the failure of the secondary pulley rotation sensor, the secondary pulley pressure is rapidly reduced, and further a demand torque to an engine is reduced according to the secondary pulley pressure which is reduced.
However, in the above conventional device, an actual torque of the engine cannot be reduced according to the secondary pulley pressure that is rapidly reduced, so that when the above control of hydraulic pressure is performed due to the failure of the secondary pulley rotation sensor, the actual torque of the engine exceeds a belt capacity capable of holding the V-belt by the secondary pulley pressure in the process wherein hydraulic pressure is reduced, and an actual belt slip occurs due to short of the belt capacity.